A novel conjugate consisting of Ricin A chain disulfide linked to the Fab' fragment of specific antibody has been constructed. The specific binding site on the antibody half of the molecule functions as a directional carrier, allowing A chain entry into the cytoplasm of cells bearing appropriate antigen, and expression of cytotoxic effects. Proposed is the construction of such a conjugate, utilizing affinity-isolated and hybridoma-derived anti-CEA (carcinoembryonic) antibody as the specific antibody. The specific cytotoxicity of such conjugates, constructed both with specific Fab' and intact IgG, will be evaluated in in vitro and in vivo tumor systems. The mechanism of intracytoplasmic entry of A chain following antibody surface binding will also be addressed.